How-to Use HVAC Display as Tachometer - ZR-1 Net Registry Forums

ghlkal · 06-30-2012

Did you know you can check engine RPM on your HVAC (C68) display? Why? Well, my tach is way off and I wanted to see how far. I ordered a Tech 1 which didn’t work, so I’m wondering how could I see how far off the tach was.

As I was troubleshooting an AC problem, I discovered that in addition to displaying HVAC fault codes, the HVAC display could show other parameters too.

In my 1990 FSM, this is on page 1C1-5

To display fault codes from the HVAC system, press and hold both [fan up] and [fan down] buttons simultaneously for 5 seconds. Pressing the [fan auto] button after that will display any HVAC fault codes (eg, I had code 09 – low Freon). Now, if you press [fan up] you can move through a number of additional parameters.

Parameter 6 is engine RPM (RPM / 25). After pressing [fan up] to display “6,” press [fan auto] to see the value.

At idle my tach showed just a tad above 1000 RPM. Parameter 6 on the HVAC showed 26. So 26 * 25 = 650 RPM.

(As a noob, I keep looking for some information that I can share to help pay back all the great info I get here – I searched and looked in solutions, and I didn’t see this … but, it’s probably posted and many of you may know this)

SteelBlueZR1 · 07-01-2012

Yes, I just found this too, and I was surprised that my tach was dead on accurate at idle 3k and 5k too. I believe parameter 16 is engine temp in Celsius too.

Z Factor · 07-01-2012

Reputation points added for an informative post.

scottfab · 07-01-2012

One parameter that is not in the table of the 90MY FSM is #16 which is the water temp in deg C.

ghlkal · 07-01-2012

Quote:

Originally Posted by SteelBlueZR1

I believe parameter 16 is engine temp in Celsius too.

Quote:

Originally Posted by scottfab

One parameter that is not in the table is #16 which is the water temp in deg F.

It's either temp in *F / 2 or temp in *C ... I wonder how we can find out?

Quote:

Originally Posted by Z Factor

Reputation points added for an informative post.

Here's the page from my FSM for the published parameters ...

scottfab · 07-01-2012

Quote:

Originally Posted by ghlkal

It's either temp in *F / 2 or temp in *C ... I wonder how we can find out?

What is the Serial number of your 90?
Mine reads in F.
But I will double check later this week.
I made a note the manual that it was F.

CamP · 07-01-2012

My '90 reads in Celcius. It's a late production car.

ghlkal · 07-01-2012

Quote:

Originally Posted by scottfab

What is the Serial number of your 90?

#2180

The display wasn't showing a temp in F, it appeared to be about 1/2 the temp gauge reading. So it could be in C ...

efnfast · 07-01-2012

Hey, that's pretty cool. Bonus, my tach appears to be right on.

VetteMed · 07-01-2012

Here's the chart from the 1991 FSM

Parameter 16 is actually the coolant temp in C, not F.

07-01-2012	#3
Z Factor Join Date: Dec 2003 Location: S. Florida Posts: 2,647	Re: Use HVAC display as tach Reputation points added for an informative post.

07-01-2012	#4
scottfab Join Date: Jan 2009 Location: Portland Oregon metro area (Washington side) Posts: 3,193	Re: Use HVAC display as tach One parameter that is not in the table of the 90MY FSM is #16 which is the water temp in deg C. __________________ Scott Vett owner since 1979._It's about the car and the people Last edited by scottfab; 11-10-2012 at 11:10 PM. Reason: corrected F to C and added "90MY FSM"

07-01-2012	#7
CamP Join Date: Jun 2012 Location: fort worth Posts: 181	Re: Use HVAC display as tach My '90 reads in Celcius. It's a late production car.

07-01-2012	#9
efnfast Join Date: Nov 2011 Location: Dunbarton NH Posts: 7,555	Re: Use HVAC display as tach Hey, that's pretty cool. Bonus, my tach appears to be right on.

07-01-2012	#10
VetteMed Join Date: Mar 2009 Location: Orlando, FL Posts: 1,816	Re: Use HVAC display as tach Here's the chart from the 1991 FSM Parameter 16 is actually the coolant temp in C, not F.

06-30-2012	#1
ghlkal Join Date: Jan 2012 Location: Wisconsin Posts: 885	How-to Use HVAC Display as Tachometer Did you know you can check engine RPM on your HVAC (C68) display? Why? Well, my tach is way off and I wanted to see how far. I ordered a Tech 1 which didn’t work, so I’m wondering how could I see how far off the tach was. As I was troubleshooting an AC problem, I discovered that in addition to displaying HVAC fault codes, the HVAC display could show other parameters too. In my 1990 FSM, this is on page 1C1-5 To display fault codes from the HVAC system, press and hold both [fan up] and [fan down] buttons simultaneously for 5 seconds. Pressing the [fan auto] button after that will display any HVAC fault codes (eg, I had code 09 – low Freon). Now, if you press [fan up] you can move through a number of additional parameters. Parameter 6 is engine RPM (RPM / 25). After pressing [fan up] to display “6,” press [fan auto] to see the value. At idle my tach showed just a tad above 1000 RPM. Parameter 6 on the HVAC showed 26. So 26 * 25 = 650 RPM. (As a noob, I keep looking for some information that I can share to help pay back all the great info I get here – I searched and looked in solutions, and I didn’t see this … but, it’s probably posted and many of you may know this) Last edited by ZBrink; 01-25-2025 at 04:12 AM. Reason: Title Edit

07-01-2012	#2
SteelBlueZR1 Join Date: Aug 2011 Location: Evergreen, Co. Posts: 344	Re: Use HVAC display as tach Yes, I just found this too, and I was surprised that my tach was dead on accurate at idle 3k and 5k too. I believe parameter 16 is engine temp in Celsius too.